style: Add border radius to overflow button

This also adds the `.overflow` style class.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/598>

src/gui/views/gcal-month-cell.blp

@@ -67,6 +67,7 @@ template $GcalMonthCell: Adw.BreakpointBin {
 
       styles [
         "flat",
+        "overflow",
       ]
 
       Box {

src/theme/style.css

@@ -271,6 +271,10 @@ monthcell {
   background-color: var(--bg-color);
 }
 
+monthcell .overflow {
+  border-radius: 6px;
+}
+
 monthcell .day-label {
   border-radius: 6px;
   padding: 0px 6px 0px 6px;